Adds the dancer-rapper, 31: "I looked pregnant. When I see pictures [of me] like that and people are calling me K- Fat, I had to do something about it. At my biggest, I was over 240."

He says he packed on all the pounds due to personal stresses, including a 2006 divorce from Britney Spears. "I went through a lot of things," he says. "I definitely say that those things contributed to me gaining weight for sure."

He says his children were one of the main reasons he signed on for VH1's Celebrity Fit Club: Boot Camp, which premieres Monday. He says he couldn't keep up with them during play dates. Adds the rapper, "I could barely make it up the hill on the side of my house."

Though he can't reveal how much weight he has lost, he says he is pleased with his new shape.